The present invention relates to a motor controller for stably driving a permanent magnet motor.
For starting a permanent magnet motor, the following method is known. The method includes the steps of:                letting a current flow through a specific phase and positioning a rotor, as a positioning mode;        then driving a synchronous motor without using information of a rotational angle position of the permanent magnet motor (without conducting position feedback), raising the output frequency of an inverter gradually, and accelerating the permanent magnet motor to a certain number of rotations from the positioning state, as a synchronous operation mode; and        then conduct operation at that number of rotations or above by using an estimated value of a magnetic pole position or information of the rotational angle position obtained from a magnetic pole position sensor or the like, as a position feedback operation mode.        
When changing over the operation mode at the above-described certain number of rotations in this method, a virtual rotation position based on which a control system is generated in the synchronous operation mode state might differ from an actual rotor position remarkably or continuity of the motor output torque might not be kept between before and after the changeover. In such a case, the number of rotations might become abnormally high immediately after the changeover, or conversely abnormally low. A changeover shock involving a remarkable change in the number of rotations is thus caused. The degree of changeover shock varies depending upon the changeover method and the load condition at the time of changeover.
As one of other shocks caused when the operation mode is changed over, there is a problem of a peak current involving a current increase. As for a technique for suppressing the peak current to a low value, there is a method described in, for example, JP-A-2004-222382. As a method for determining a voltage in a synchronous operation mode in this conventional technique, a load torque is estimated on the basis of a relation that a current flowing through the permanent magnet motor decreases as the load torque increases, and a voltage corresponding to the estimated load torque is applied to the permanent magnet motor. Thereafter, changeover to a mode using information of the rotational angle position is conducted, when phase differences between three-phase phases and the rotational angle position are within a prescribed phase difference range.